The present invention relates generally to electrode frames used in electrolytic cells. More specifically, the present invention relates to an improved composite fiber reinforced plastic frame that may be employed in monopolar filter press type of membrane electrolytic cells, especially those used to produce chlorine and caustic.
Chlorine and caustic, products of the electrolytic process, are base chemicals which have become large volume commodities in the industrialized world today. The overwhelming amounts of these chemicals are produced electrolytically from aqueous solutions of alkali metal chlorides. Cells which have traditionally produced these chemicals have come to be known as chloralkali cells. The chloralkali cells today are generally of two principal types, the deposited asbestos diaphragm-type of electrolytic cell or the flowing mercury cathode-type of cell.
The development of a hydraulically impermeable membrane has promoted the advent of commercial filter press membrane chloralkali cells which produce a relatively uncontaminated caustic product. This higher purity product can obviate the need for caustic purification and concentration processes. The use of a hydraulically impermeable planar membrane has been most common in bipolar filter press membrane electrolytic cells. However, advances continue to be made in the development of monopolar filter press membrane cells which have caused increasing attention to be focused on the development of improved and more economical electrodes and electrode frames.
Early filter press membrane cells were constructed of heavy plastic frames. Typically, these cells were bipolar and utilized a solid sheet or backplate which was a divider between the cells and was fabricated integrally with the frame. Bipolar cells of this type followed well developed filter press membrane fabrication principles. The integral frame-backplate construction provided excellent stiffening of the frame structure. The backplate frequently was covered with a resin or rubber coating that was not readily attacked by the chlorinated brine. The frames for these cells were molded from hard rubber and filled with polypropylene or molded of polyester or any other material that was chemically inert. Frequently, the anode frame was formed of these materials while the cathode frame either was made from the same materials or was formed from steel.
The bipolar filter press membrane cell frames tended to be limited in size for several reasons. These included the high cost for very large molds and the warping that tended to occur in the heavy plastic frames when the frames were subjected to operating temperatures during actual cell use. Additionally, the plastic parts employed in these cells tended to have a high coefficient of expansion compared to the metal parts. This resulted in a disparity in expansion between the cell parts during operation that tended to cause distortion. Also, the filled plastic frames were susceptible to corrosion by the chlorine, especially in the filler material. Lastly, the presence of calcium and magnesium in these plastic frames was found to be inappropriate when membranes were used because of the adverse affect on membrane life.
Thus, monopolar filter press membrane cells, as well as bipolar filter press membrane cells, normally have employed metal frames. Typically, these metal frames have used titanium in the anodic electrode and iron or nickel in the cathodic electrode. This metal frame construction offered the advantages of high strength, small cross section of structural members, corrosion resistance, resistance to warping, large size and compatibility with metal electrode surfaces. However, the single most notable disadvantage of metal frames is their very high fabrication cost. Metals such as titanium and nickel, and the fabricating facilities as necessary to produce the electrode frames, are particularly susceptible to the soaring costs associated with high technology.
Therefore, attempts continue to employ plastic frames in filter press membrane cells that will give the advantages that metal frames offer without the high costs. Pultruded members of fiberglass polyester resin offer the advantages of low cost, low coefficient to thermal expansion similar to those of metal, and high strength. However, this type of plastic frame construction is deficient because of inadequate corrosion resistance and the difficulty of obtaining strong, leak-free, corrosion-resistant corner joints.
Where composite fiber reinforced plastic frames with resin impregnated glass fiber core material wound within a chemically resistant thermoplastic liner have been employed, high stress levels have been found to exist in the glass fibers at the corners of the frames. This high stress is the result of the resin being pressed off of the glass fibers at the frame corners because of the manner in which the glass fibers are wound onto the frames. Frames are rotated on a rotatable jig.
The foregoing problems are solved in the design of the composite fiber reinforced plastic frames of the present invention.